Axial piston compressor

ABSTRACT

In an axial piston compressor for a refrigerant, in particular for a vehicle air conditioner, with a housing ( 12 ) in the interior of which is disposed at least one bearing ( 22, 24, 26, 48, 56, 58, 60, 64 ), with an output conduit ( 34 ) for the compressed refrigerant, and with a lubricant that is present in the interior of the housing ( 12 ), the lubrication is to be improved. For this purpose there is provided in the housing at least one lubricant channel to conduct lubricant to the bearing, as well as a lubricant separator ( 36 ) that is connected to the output conduit ( 34 ) for the refrigerant and incorporates a collection chamber ( 40 ) that contains the lubricant separated from the compressed refrigerant and is connected to the lubricant channel ( 46 ) by a feed line ( 42 ), such that the lubricant is propelled out of the lubricant separator and into the lubricant channel entirely on the basis of the pressure difference between the compression pressure of the lubricant and the interior pressure of the housing.

DESCRIPTION

The invention relates to an axial piston compressor for a refrigerant,in particular for a vehicle air conditioner, with a housing in theinterior of which at least one bearing is disposed, with an outletconduit for the compressed refrigerant, and with a lubricant presentwithin the interior of the housing.

Such an axial piston compressor is known, for example, from the Germanpatent DE 196 21 174 A1. It is used to compress the refrigerant in avehicle air conditioner and serves to suck in the refrigerant from aheat-transfer compartment in which it evaporates while taking up heat atlow pressure, and to compress it to a higher pressure at which it ispassed into another heat-transfer compartment where, while releasingheat, the refrigerant is returned to the liquid state and/or cooled.

Such compressors have been produced in a great variety of constructions;for various reasons, the most generally accepted axial pistoncompressors are those that operate with a swash plate. In thisconstruction the axial movement of the piston is generated by a swashplate, tilted relative to the drive shaft at an angle that can becontrolled. The pistons are connected to the swash plate so that theycannot be shifted by forces of compression or traction; because thecylinders within which the pistons move are fixed in place while theswash plate is in motion, as a coupling mechanism between the swashplate and the pistons there are provided either sliding blocks supportedin sliding bearings situated on the pistons, or a wobble plate withpiston rods seated on sliding bearings on the pistons. When the pistonsare directly connected to the swash plate, on each piston there areformed two hemispherical bearings in which the two sliding blocks aredisposed so that each contacts a slideway, one on one side of the swashplate and the other on the other side. In contrast, when a wobble plateis used, it is mounted on the swash plate so that it can be rotated withrespect thereto, so that what is transmitted to the wobble plate is onlythe angle of tilt of the swash plate but not its rotational movement.The piston rods are seated on both the wobble plate and the piston byway of ball-and-socket joints.

In the case of axial piston compressors employed in motor vehicles, itis impossible to lubricate their components by circulating oil with apump. For one thing, the axial piston compressor can in somecircumstances become much more expensive when a lubricant pump isincluded. Furthermore, such a pump impairs performance, which in thecase of an axial piston compressor for cooling vehicles, which tends tohave a low output at best, is more significant than in the case of ahigher-output axial piston compressor. Finally, the volume of theoverall structure would be considerably increased by providing a pumpthat has to suck the lubricant in from an oil sump, and by the sumpitself. For all these reasons the lubrication in the interior of thehousing is accomplished not by a pump-driven circulation of oil butrather by a mist of oil generated within the housing. Furthermore, it isknown from the European patent application 0 738 832 that an oil sumpcan be employed to collect oil droplets that are produced in theinterior of the housing. This oil sump is connected to a reservoir inthe interior of the housing by way of a lubricant-oil channel. Becausethe oil sump is positioned above the corresponding reservoir, the oilflows into the reservoir under the influence of gravity.

The situation is quite similar in the case of the axial pistoncompressor according to the patent DE 198 21 265 A1. There, too, gravitycauses oil to drop onto moving parts within a wobble-plate chamber.Accordingly, this construction also allows the bearings to be lubricatedwithout applying pressure.

The same applies to the construction according to the patent U.S. Pat.No. 4,283,997. Although in this case oil is separated out on thehigh-pressure side and sent from there to the bearings of the movingparts of the axial piston compressor, there is a throttle behind the oilseparator on the high-pressure side, and furthermore the oil channelopens into an oil-collecting chamber connected to the drive-mechanismchamber by way of a radial bearing for the drive shaft. Therefore thepressure of the lubricant oil falls nearly to the same level as that inthe drive-mechanism chamber, with the consequence that the supply oflubricant oil to the bearings is considerably reduced. Thisconstruction, too, is thus distinguished by a nearly unpressurizedapplication of lubricant.

However, the previously provided means of lubrication, by a mist of oilor by the application of lubricant without pressure, is not satisfactoryunder all operating conditions. In particular in the case of slidingbearings, which involve only a slight amount of oscillatory relativemovement, the lubrication can be deficient because it is impossible toprovide enough lubricant in the form of a mist.

The objective of the invention is thus to lubricate the bearings of anaxial piston compressor of the kind described above with pressurizedoil, reliably and by the simplest means, with no need for a separate oilpump. Nevertheless, the lubrication thus achieved is designed to be ofhigher quality than the lubrication of bearing sites by the lubricantmist present in the interior of the housing.

ADVANTAGES OF THE INVENTION

An axial piston compressor of the kind cited above with the featuresgiven in the characterizing part of claim 1 offers the advantage that apressurized lubricant circulation is created that can conduct therequired lubricant to the bearing sites in the interior of the housing.Expressed simply, this lubricant circulation is based on recycling theamount of lubricant that unavoidably leaves the axial piston compressoralong with the compressed refrigerant, as well as on utilizing thepressure difference between the compressor side and the interior of thehousing of the axial piston compressor. Because the pressure on thecompressor side is much greater than that in the interior of thehousing, the resulting throughput of lubricant is very high, with noneed to provide a separate energy source or even a pump for the purpose.The lubricant separator provided in accordance with the invention is acomparatively simple component, which involves no great expenditure.

Within the lubricant circulation so created a suitable throttlingensures the resistance necessary to prevent the lubricant from flowingout of the separator too rapidly. Such throttling results automatically,for example, when a sliding bearing is being lubricated; the narrow gapin the bearing limits the rate at which lubricant can flow through. Incontrast, when a roller bearing is being lubricated, in somecircumstances a cover plate must be used to restrict the flow crosssection by a suitable amount.

The construction in accordance with the invention thus is characterizedby the fact that even the last bearing to be supplied with lubricant issupplied under high pressure, i.e. compression pressure. This isachieved because the bearings themselves represent throttling sitestowards the low-pressure side; that is, they are either sealed offtowards the low-pressure side or on the basis of their construction—thisapplies in particular to sliding bearings—constitute an extremelyefficient throttle towards the low-pressure side. The oil pressure isapplied to the bearing or gap in the bearing. With respect to theirsupply of lubricant oil, the bearings are arranged in series such thatthe pressure of the oil supplied to even the last bearing is nearlyunchanged, i.e. is still high. Only on the basis of this construction isit possible to replace an oil pump by an oil separator on thehigh-pressure side, without impairing the supply to the bearings of oilunder high pressure.

When both sliding and roller (in particular needle) bearings areemployed, preferably the oil is first sent to the sliding bearings with“narrow gaps”, because sliding bearings on account of their constructionserve as efficient throttling sites towards the low-pressure side. Incase the construction is such that roller or needle bearings must comefirst in the series, these should be sealed off towards the low-pressureside. In the sense of the above explanation, at the core of the presentinvention is the fact that the lubricant is propelled from the lubricantseparator to the lubricant channel entirely on the basis of the pressuredifference between the compression pressure of the refrigerant and theinternal pressure of the housing, and is conducted by the channel undercorresponding pressure to the bearings. This means that all the bearingsare supplied with lubricant under high pressure, with no need for aseparate oil pump.

In this arrangement, the lubricant separator is disposed on the pressureside of the circulation, either between the compressor and apressure-side heat exchanger or between the pressure-side heat exchangerand an expansion valve.

According to one preferred embodiment of the invention, the feed line isprovided with a controllable valve. With this valve the feed line can beclosed while the axial piston compressor is not operating, so that thehigh pressure on the compressor side cannot press the lubricant that iswithin the collection chamber out into the housing of the axial pistoncompressor and thereby eventually empty the collection chamber. If thatwere to happen, no lubricant would be available when the axial pistoncompressor was put into operation. On the other hand, if the valve isopened as soon as the axial piston compressor becomes operative, thecompressor can be adequately lubricated immediately, by the lubricantthat has accumulated in the collection chamber.

The controllable valve could be eliminated if outflow of the lubricantfrom the collection chamber were counteracted by a sufficientresistance, namely the internal resistances and throttling sites of thesystem. That is, if the compressor is only briefly inoperative, there isnot enough time for equilibration of the pressures on the high- andlow-pressure sides of the compressor. Hence when the compressor isturned on again, it must work against a high pressure; in compensation,however, lubricant is immediately available. On the other hand, afterthe compressor has been inactive for a long time, so that the pressureshave equilibrated and pressurized lubricant is not deliveredimmediately, it takes some time for the pressure to be built up; thatis, the compressor is initially not working under a heavy load, so thatcomplete lubrication is not immediately required. As the load on thecompressor increases, the lubrication improves accordingly.

Alternatively, the controllable valve could be replaced by a throttlingsite, if it is ensured that the pressure equilibration during compressorstandstill is brought about primarily by other parts of the circulationsystem, for example by a separate valve.

Instead of a separate throttling site, a throttle conduit could be used,in which the required throttling is produced by the various pressurelosses in the lubricant supply system, in particular by channels in thecompressor.

Preferably an overflow conduit is provided, which connects the lubricantseparator to the interior of the housing. By this means excess lubricantthat has accumulated in the collection chamber can be removed whennecessary. For this purpose a controllable valve can be provided in theoverflow conduit, which is opened in dependence, for example, on thesignal from a sensor that detects the level of the contents in thecollection chamber.

According to one embodiment of the invention it can be provided that thelubricant separator is integrated into the housing. This enables aparticularly compact construction.

Alternatively, it can be provided that the lubricant separator isseparate from the housing and the feed line acts as a lubricant cooler.This ensures that the lubricant fed back to the bearing sites, which hasbeen warmed by the refrigerant during the compressor stroke, is returnedto its initial temperature.

According to one preferred embodiment, a drive shaft with an axialdistributor bore is provided. An axial distributor bore in the driveshaft enables nearly all the important bearing sites in the interior ofthe housing of the axial piston compressor to be reached with aparticularly simple arrangement. The complexity in this case isdistinctly less than in the case of an arrangement such that individuallubricant channels are provided for all the bearing sites in thecompressor housing.

Preferably the distributor bore opens at an end face of the drive shaftsituated in the interior of the housing, i.e. the face at the endopposite to the drive end of the drive shaft. Given an axial deliveryroute of the lubricant, because of the low circumferential velocities asmall axial sealing element can be used, so that the whole structure canbe made compact. According to a preferred embodiment a swash plate isprovided that is disposed on the drive shaft so that it can move along asliding bearing, and in the region of the sliding bearing a branch boreis provided in the drive shaft to connect the sliding bearing with thedistributor bore. The sliding bearing is very difficult to lubricate bymeans of the lubricant mist present in the interior of the housing; thebranch bore makes it possible to conduct the necessary amount oflubricant to the sliding bearing. The amount thus conducted can bedetermined by adjusting the cross-sectional area of the branch bore.

Preferably a supply bore is formed within the swash plate, into whichlubricant passing through the sliding bearing flows, so that slidingblocks in contact with the swash plate can be supplied with lubricant byway of the supply bore. By this means the sliding blocks, which movewith only a slight oscillatory component and hence are difficult tolubricate by means of a lubricant mist, are supplied in a targetedmanner with pressurized lubricant.

According to a preferred embodiment the drive shaft is seated in atleast one subassembly comprising radial and axial bearings, thissubassembly being supplied with lubricant by a branch bore from thedrive shaft such that the lubricant flows first through the radialbearing and then through the axial bearing. This sequential arrangementof the bearings with respect to the lubricant flow makes it possible forboth bearings to be lubricated in a relatively uncomplicated manner.Because the space available limits the size of the radial bearing, thisbearing is likely to have the shortest working life and hence is thefirst to be supplied with lubricant; the lubricant flowing out of theradial bearing is then sent to the axial bearing. As is preferably thecase, this order of lubrication can be produced by sealing disks thatform leakage gaps of specified dimensions. By suitably dimensioning thesites through which the lubricant passes, the function of anon-controllable valve can be simulated, so that when the compressor isnot operating, it is impossible for too much lubricant to be transferredfrom the separator into the compressor.

According to one preferred embodiment of the invention, CO₂ is used asthe refrigerant. Apart from various technical advantages of CO₂ incomparison to conventionally employed refrigerants such as R134a, an airconditioner with CO₂ as refrigerant operates at a much higher pressurethan does an air conditioner with a conventional refrigerant. When CO₂is used, the suction pressure is about 50 bar and the compressionpressure, about 120 bar. In contrast, the suction pressure for therefrigerant R234a is about 5 bar and the compression pressure, about 20bar. As a result, when CO₂ is used as the coolant the pressuredifference between the lubricant separator and the interior of thehousing of the axial piston compressor is much greater than inconventional axial piston compressors, namely about 70 bar as comparedwith 15 bar. This increase in pressure differential in accordance withthe invention produces an improved supply of lubricant to the bearings.

Advantageous embodiments of the invention will be apparent from thesubordinate claims.

DRAWINGS

In the following the invention will be described with reference tovarious embodiments illustrated in the attached drawings, wherein

FIG. 1 is a schematic sectional view of an axial piston compressoraccording to a first embodiment of the invention;

FIG. 2 shows a schematic section of an axial piston compressor accordingto a second embodiment of the invention; and

FIG. 3 shows a schematic section of an axial piston compressor accordingto a third embodiment of the invention.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

In FIG. 1 an axial piston compressor according to a first embodiment isshown schematically. It comprises a drive shaft 10 seated in a housing12. To the drive shaft 10 there is nonrotatably connected a swash plate14, which can be pivoted between a position in which it is approximatelyperpendicular to the long axis of the drive shaft 10 and a maximallytilted position shown in FIG. 1. The position occupied by the swashplate 14 while in operation is adjusted in dependence on the differencebetween the intake pressure of the compressor and the pressure in theinterior of the housing 12 as well as on the pretensioning of a spring16 that can shift the swash plate along the drive shaft 10; the swashplate is braced against a holder 18 so that as it is shifted along thedrive shaft, it is also pivoted.

A wobble plate 20 is rotatably seated on the swash plate by means ofradial and axial roller bearings 22, 24. Engaged with the wobble plate20 are several ball-and-socket joints 26, each of which provides atension- and pressure-resistant connection between a piston 28 and thewobble plate 20. Each piston 28 can move within a cylinder 30, thecentral axis of which is parallel to the long axis of the drive shaft10. In the drawing only two pistons are shown; in fact, the compressorcan contain as many as seven pistons.

When the drive shaft 10 is put into rotation and the swash plate istilted with respect to the drive shaft, each piston 28 moves back andforth within its cylinder 30. This movement can be utilized to compressa refrigerant, for example CO₂. Under evaporative or suction pressure,the refrigerant is extracted from an input conduit 32, and undercondensation or evaporation pressure it is sent into an output conduit34. During the compression process the refrigerant takes up smallamounts of a lubricant that is present in the interior of the housingand is also deposited on the inner wall of the cylinder 30.

The output conduit 34 opens into a lubricant separator 36. Thiscomprises, firstly, a separator compartment 38 in which the flowvelocity of the refrigerant, which is present as a pressurized gas, islowered and as a result the lubricant is separated out by the force ofgravity; in addition it comprises a chamber 40 in which the separatedlubricant is collected. The lubricant contained in the collectionchamber 40 is under the pressure of the refrigerant. From the separatorcompartment 38, the compressed refrigerant is conducted to a heatexchanger through a compressor conduit 39.

As an alternative to a gravity separator, in principle any generallyused means of separation can be used to implement the lubricantcirculation.

Attached to the lowest part of the collection chamber 40 is a feed line42, which is provided with a controllable valve 44. The feed line 42leads to a supply channel 46 in the housing 12, which opens into aradial bearing 48 for the drive shaft 10. To the separator compartment38 is attached an overflow conduit 43, provided with a valve 45. Openingof the valve 45 enables an excess volume of lubricant contained in thecollection chamber 40 to be returned to the housing.

The drive shaft 10 is provided with a distributor bore 50 that extendsaxially and is connected by way of a radially extending supply bore 52to the radial bearing 48. The drive shaft 10 is further provided withtwo radially extending branch bores 54, one of which is associated witha sliding bearing 56 by means of which the swash plate is seated on thedrive shaft 10, while the other is associated with a radial bearing 58which, together with an axial bearing 60, supports the end of the driveshaft 10 disposed in the interior of the housing 12, namely the endopposite the drive end of the shaft.

When during operation of the axial piston compressor described here thevalve 44 of the feed line 42 is opened, the lubricant contained in thecollection chamber 40 flows through the feed line 42 to the supplychannel 46, because of the difference between the pressure in theseparator compartment 38 and the interior of the housing 12. From thesupply channel it flows into the distributor bore 50 of the drive shaft10, by way of the radial bearing 48 and the supply bore 52. From thedistributor bore it can reach the various bearing sites in the interiorof the housing by way of the branch bores 54. In this way the slidingbearing 56 as well as the subassembly consisting of radial bearing 58and axial bearing 60 are lubricated. The radial bearing 58 is soconstructed that the lubricant that has flowed through it continues tothe axial bearing. For this purpose, the radial bearing can beintegrated into the housing in such a way that a projection of thehousing, together with the rotating drive shaft, forms a narrow gap thatallows only as much lubricant to escape as can ensure an acceptablelubricant pressure throughout the entire “serial arrangement” of bearingsites.

That the lubricant introduced to the interior of the housing will bereturned to the lubricant separator is ensured by the fact that, becauseof the rotation of the components of the axial piston compressor, a mistof lubricant is always present in the interior of the housing. This isdeposited on the inner wall of the cylinder 30, and from there it iscarried by the compressed refrigerant into the lubricant separator.

In FIG. 2 a second embodiment of an axial piston compressor isdiagrammed. For the components already described in the first embodimentthe same reference numerals are used here, so that the explanationsgiven above also apply here.

In contrast to the first embodiment, in the second embodiment thelubricant is conducted axially in the distributor bore 50 within thedrive shaft 10, having been introduced at the end of the drive shaftthat is on the right in FIG. 2. For this purpose, on the end face of thedrive shaft 10 a sealing element 62 is provided, which can have smalldimensions because the circumferential velocity there is so low.

In this embodiment, a branch bore 54 is now provided in the region ofthe radial bearing 48 associated with the drive end of the drive shaft,so that this bearing is reliably provided with lubricant. From thisbearing the lubricant flows to an axial bearing 64, which supports theholder 18.

FIG. 3 is a diagram of an axial piston compressor according to a thirdembodiment. Here, again, for known components the same referencenumerals are used as in FIG. 1, so that for these reference is made tothe explanations given above.

Here the lubricant is conducted radially, as in the first embodiment,but it is introduced in the region of the radial bearing 58. From thereit can flow through the distributor bore 50 to the sliding bearing 56and the radial bearing 48.

A difference from the first embodiment is that in the third embodiment asupply bore 66 is provided both in the swash plate 14 and in the wobbleplate 20. Hence the lubricant emerging from the branch bore 54 can passthrough the sliding bearing 56, the radial bearing 22 and the wobbleplate 20 to reach the ball-and-socket joints 26 and lubricate thelatter, in particular the sliding blocks disposed in the ball-and-socketjoints.

It is also possible to supply the pistons 28 in the cylinders 30 withpressurized oil, so as to produce a better film of lubricant in theregion of the friction pairing there, which can be regarded as a slidingbearing. For this purpose, a lubricant pocket is formed in the cylinderface, which is supplied with lubricant through a suitable channel. Thenarrow gap between cylinder and piston ensures the required throttlingof the lubricant throughput.

LIST OF REFERENCE NUMERALS

-   10 Drive shaft-   12 Housing-   14 Swash plate-   16 Spring-   18 Holder-   20 Wobble plate-   22 Roller bearing-   24 Roller bearing-   26 Ball-and-socket joint-   28 Piston-   30 Cylinder-   32 Input conduit-   34 Output conduit-   36 Lubricant separator-   38 Separator compartment-   39 Compressor conduit-   40 Collection chamber-   42 Feed line-   43 Overflow conduit-   44 Valve-   45 Valve-   46 Supply channel-   48 Radial bearing-   50 Distributor bore-   52 Supply bore-   54 Branch bore-   56 Sliding bearing-   58 Radial bearing-   60 Axial bearing-   62 Sealing element-   64 Axial bearing-   66 Supply bore

1. Axial piston compressor for a refrigerant, in particular for avehicle air conditioner, with a housing (12) in the interior of which isdisposed at least one bearing (22, 24, 26, 28, 30, 48, 56, 58, 60, 64),with an output conduit (34) for the compressed refrigerant, and with alubricant that is present in the interior of the housing (12),characterized in that in the housing at least one lubricant channel (46)is provided to conduct lubricant to the bearing, as well as a lubricantseparator (36) that is connected to the output conduit (34) for therefrigerant and incorporates a collection chamber (40) to contain thelubricant separated from the compressed refrigerant, and in that a feedline (42) having a controllable valve (44) is provided to connect thecollection chamber to the lubricant channel (46), wherein the lubricantis propelled out of the lubricant separator and into the lubricantchannel entirely on the basis of the pressure difference between thecompression pressure of the lubricant and the interior pressure of thehousing, and is conducted through the channel to said bearing (48, 56,58, 60, . . . ) under correspondingly high pressure.
 2. Axial pistoncompressor according to claim 1, characterized in that an overflowconduit (43) is provided, which connects the lubricant separator (36) tothe interior of the housing.
 3. Axial piston compressor according toclaim 2, characterized in that the overflow conduit (43) is providedwith a controllable valve (45).
 4. Axial piston compressor for arefrigerant provided with a drive shaft (10) containing an axialdistributor bore (50), in particular for a vehicle air conditioner, witha housing (12) in the interior of which is disposed at least one radialand axial bearing (22, 24, 26, 28, 30, 48, 56, 58, 60, 64), with anoutput conduit (34) for the compressed refrigerant, and with a lubricantthat is present in the interior of the housing (12), characterized inthat the drive shaft is supported by at least one subassembly consistingof radial bearing (58) and axial bearing (60) provided with sealingdiscs that form a specified leakage gap (10) and that this subassemblyis supplied with lubricant by a branch bore (54) in the drive shaft,such that the lubricant flows first through the radial bearing and thenthrough the axial bearing.